1. Technical Field
The present invention relates to a pulse signal generating device that generates a pulse with a restored period by detecting period abnormalities in an output pulse of an encoder, a transport device, an image forming apparatus, and a pulse signal generating method.
2. Related Art
A known image forming apparatus, such as a printer, has a configuration in which a recording head performs printing on paper (target) transported in a transport direction. In this case, it is necessary to discharge ink droplets at proper timing according to the paper transport position. For this reason, a print reference signal is generated in synchronization with the transport speed of paper on the basis of an output signal (pulse signal), which is output from an encoder in synchronization with the paper transport speed, and the discharge timing is controlled on the basis of the print reference signal.
For example, JP-A-11-245383 discloses a printer (image forming apparatus) that uses a transport belt as a paper transport unit. A mark for detecting the speed and position is set on the transport belt, and a character or an image is printed on the paper by reading the mark with an encoder and discharging ink on the basis of the encoder signal.
As disclosed in JP-A-11-245383, in a printer using a paper transport unit such as a transport belt, ink droplets are discharged for every desired pitch by detecting the amount of movement of recording paper from the amount of movement of a belt in a condition where the amount of movement of recording paper is assumed to be equal to the amount of movement of the belt. Since discharge of ink droplets is performed in synchronization with an encoder signal having a pulse with the same distance as a print pitch, high-quality printing capable of suppressing the positional deviation of landing ink droplets becomes possible even if there is a speed variation in the transport unit. However, since encoder signals need to be output continuously at the same pitch in the method disclosed in JP-A-11-245383, the following problems occur.
If the circumferential length of the recording paper transport unit (transport belt) is not integral multiples of the print pitch, a discontinuous portion occurs in an output signal of a linear encoder, which causes image deterioration. Moreover, when ink mist or paper particles adhere to the linear encoder disposed on the transport belt or the linear encoder is damaged, lack of a pulse occurs in the encoder signal. Also in this case, image deterioration occurs.
A device for solving such a problem is disclosed in JP-A-2003-280484 (for example, paragraphs [0023] to [0061] in the specification and FIGS. 1 and 9 to 22) and JP-A-2005-350195 (for example, paragraphs [0041] to [0053] in the specification and FIGS. 6 to 11), for example.
The printer disclosed in JP-A-2003-280484 has a configuration where the motor speed control is performed by a PLL using a linear encoder, a unit that detects a discontinuous portion of a linear encoder is provided, and a speed and position control unit is changed on the basis of the detection result of the discontinuity detection unit. In a discontinuous portion, an output interval average value of an output signal measured in a continuous portion is used. Moreover, a control method in case of using two sensors is described as measures against a case where a discontinuous portion is long, such that it is prevented that the belt speed cannot be detected for a long period of time.
JP-A-2005-350195 discloses a configuration where two sensors for detecting a mark are provided such that positions of objects to be detected are different and an output signal, which is used for motor control for controlling the transport speed of a belt constantly, is switched to an output signal of the other sensor when a discontinuous portion of a mark, such as a joint of the transport belt, or lack of a pulse caused by dust or damage is detected on the basis of an output signal of one sensor. Moreover, in order to suppress the phase difference between signals at the time of sensor switching, a configuration where an interpolation processing portion generates a high-resolution signal by dividing a measured period or using the same clock so that a signal error (phase difference) is reduced is adopted. Moreover, JP-A-2003-280484 and JP-A-2005-350195 disclose a method of counting a period of a sensor signal (encoder signal) with a base clock and determining that there is a discontinuous portion (lack of a pulse of an encoder signal) of a mark when the pulse period of the sensor signal reaches a fixed threshold value or more.
Furthermore, JP-A-2004-21236 discloses a method of performing a motor speed control using a linear encoder provided on a belt. According to this method, a unit that detects the moving speed of an encoder mark, calculates and acquires the control amount of a belt that reaches the target speed set beforehand on the basis of the moving speed, and controls the speed of the belt with the control amount before the next encoder mark appears is provided. When an encoder mark is not clear due to a dirt or damage, operation and acquisition of the control amount are not performed, and the control amount at a previous mark is held and the speed control is performed using the held control amount.
In addition, a positional deviation control device that detects an error, such as unclearness, of an encoder mark is described in JP-A-2004-21236. The positional deviation control device performs error detection using the following conditional expression (a).0.99×To≦t≦1.01×To  (a)                (where, t: detection signal period, To: reference detection time To=Do/Vo, Do: gap between encoder marks, Vo: reference moving speed)        
However, in the technique disclosed in JP-A-2003-280484, a method of improving the printing accuracy by performing the motor speed control such that the speed of the transport belt is constant is adopted. Accordingly, there was a possibility that a controller for motor control would be complicated and the control would not be constant. Furthermore, in the technique disclosed in JP-A-2005-350195, a method of improving the printing accuracy by performing the motor speed control such that the speed of the transport belt is constant is adopted. Accordingly, there are problems that the control is variable and the device needs to adjust a control gain and the like, for example.
Moreover, the conditional expression (a) disclosed in JP-A-2004-21236 is an effective means only in case of performing the speed control. In case of performing printing in synchronization with an encoder signal as disclosed JP-A-11-245383, the reference detection time To (threshold value for period error detection) becomes a fixed value. In this case, according to the individual difference of the device, a speed distribution central value of a belt changes. Accordingly, in devices shifted up and down with respect to a speed distribution central value where the speed distribution central value is set, there was a problem that even in the case of a normal signal, the detection signal period t exceeds a threshold value of a lower limit (=0.99·To) or upper limit (=1.01·To) and the normal signal is detected as an error. In order to prevent such a problem, a method of calculating the speed central value for every apparatus at the time of shipment may be considered. In this case, however, there are disadvantages that it is not possible to cope with an increase in cost or a temporal change.